No, it didn’t take 2/3rds to vote them in, why should the leave threshold be higher?
Actually, in the 1975 referendum on the UK in the EU, 67% voted for the UK in the EU
As the Canadian and Quebec government learned in Quebec, if the vote is close to evenly split on a complex question, then in reality there is too much opposition to make a huge decision such as the break up of a country happen. Having a larger percentage (66.7%) with a straightforward question makes it much easier to leave and also discourages referendums by groups who want to leave.
David Cameron should have learned the lessons of what happened on Canada and, if a second referendum takes place, the British would be wise to follow Canada’s lead.